The present invention relates to a photodetector sensitive to infrared radiation. In particular, the present invention provides for a diffraction grating coupled infrared photodetector with improved sensitivity by decreasing the thermal leakage current and thus the noise.
In the field of infrared (IR) imaging, the current objective is to provide large area focal plane arrays at low cost with high performance. InSb, HgCdTe, and quantum well infrared photodetector (QWIP) technologies have demonstrated high performance large area focal plane arrays. Each of these technologies has various strengths and weaknesses. InSb photodetectors offer high performance and ease of fabrication, but must be cooled to approximately 80 K. HgCdTe photodetectors can be designed to operate in the middle wavelength IR (MWIR) corresponding to a wavelength range of 3 to 5 xcexcm, the long wavelength IR (LWIR) corresponding to a wavelength range of 8 to 12 xcexcm, or the very long wavelength IR (VLWIR) corresponding to a wavelength range of greater than 12 xcexcm. However, HgCdTe photodetectors require very tight tolerances in material and fabrication uniformity to ensure high performance. QWIP photodetectors have been demonstrated in the MWIR, the LWIR, and the VLWIR while requiring only moderate tolerances in both material and fabrication uniformity.
Because photodetectors fabricated from HgCdTe have the greatest potential performance at a given operating temperature, significant time and effort have been expended to improve the HgCdTe starting material and fabrication process. While progress has been made, the cost of implementing these improvements is significant. Thus, there exists a need for a design that places fewer and/or less stringent requirements upon the starting material and/or the fabrication process.
In one embodiment of the present invention, a photodetector comprises a plurality of intersecting elongate IR absorbing elements, an enlargement of a portion of one of the elongate IR absorbing elements to form a collector element, a carrier collector, a first electrical contact electrically connected to the carrier collector, a second electrical contact connected to the elongate IR absorbing elements, and a reflector. The plurality of intersecting elongate IR absorbing elements form a two-dimensional diffraction grating that is designed to resonate at the IR wavelength of interest. The collector element may be a number of shapes including a circle, an oval, or a diamond. The carrier collector is formed within a portion of the collector element.
In another embodiment of the present invention, the collector elements are formed midway between the intersections of the IR absorbing elements. Another embodiment of the present invention includes collector elements that are formed at both the intersections of the IR absorbing elements and midway between the intersections of the IR absorbing elements.
In another embodiment of the present invention, the diffraction grating is designed to resonate at two different wavelengths. The first wavelength resonates in a first direction of the grating while the second wavelength resonates in a direction normal to the first direction. The wavelengths are within ten percent of each other, thereby allowing a broader spectral response.
In each of these embodiments, the IR radiation is absorbed in the IR absorbing elements and the resultant electrical carriers are attracted to the nearest carrier collector. These electrical carriers are sensed in an external circuit via the first and second contacts. The electrical carriers may be sensed as a current if the external circuit is of low impedance or as a voltage if the external circuit is of high impedance.
Photodetectors comprising a single element, a one-dimensional line array of photodetectors, or a two-dimensional area array of photodetectors are envisioned. Depending upon the specific embodiment, a number of different material systems may be used to form the IR absorbing elements, the collector elements, the carrier collectors, and the first and second electrical contacts. These material systems include II-VI semiconductor compounds that include elements from group II and group VI of the periodic table and III-V semiconductor compounds that include elements from group III and group V of the periodic table.